introduction to semiconductors material chapter 1 (week 2)
TRANSCRIPT
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INTRODUCTION TOSEMICONDUCTORS MATERIAL
Chapter 1 (Week 2)
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1.3 Covalent Bonding
1. Covalent bonding occurs when pairs of electrons are shared by non-metal
atoms.
2. When atoms combine into molecules to form a solid material, they arrange
themselves in a fixed pattern called a crystal – atoms within the crystal
structure are held together by covalent bonds (atoms share valence
electrons) .
3. Atoms are electrically stable when their valence shells are complete or fully
occupied.
4. An intrinsic crystal is one that has no impurities.
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1.3 Covalent Bonding (cont.)
• The number of covalent bonds is equal to eight minus the group number in the periodic table.
• Group number = number of electrons in the valence shell.EKT 102: Basic Electronic Engineering
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1.3 Covalent Bonding (cont.)
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Covalent bonding – holding atoms together by sharing valence electronsSemiconductor atoms bond together to form a solid material = crystal
Sharing of valence electron produces the covalent bond
To form Si crystal
Figure 12: Illustration of covalent bonds in silicon crystal
(c) Covalent bonds in a silicon crystal
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1.4 Conduction in Semiconductor
• The electrons of an atom can exist only within prescribed energy bands.
• Each shell corresponds to a certain energy band and is separated from adjacent shells by band gaps - no electrons can exist.
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Figure 13: Energy band diagram for an unexcited (no external energy) atom in a pure (intrinsic) Si crystal.
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1.4 Conduction in Semiconductor (cont.)
When an electron jumps to the conduction band, a vacancy (called a hole) is
left in the valence band within the crystal.
• Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band.
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Figure 14: Creation of electron-hole pairs in asilicon crystal
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1.4 Conduction in Semiconductor
• The number of free electrons (also known as conduction electrons) is equals to the number of holes in the valence band.
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Figure 15: Electron-hole pairs in a silicon crystal. Free electrons are being generatedcontinuously while some recombinewith holes.
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1.4 Conduction in Semiconductor (cont.)
Electron Current
• When a voltage is applied across a piece of intrinsic silicon, the thermally generated free electrons in the conduction band, which are free to move, are now easily attracted toward the positive end.
• The movement of free electrons in a semiconductive material is called electron current.
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Figure 16: Electron current in intrinsic silicon isproduced by the movement of thermally generated free electrons.
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1.4 Conduction in Semiconductor (cont.)
Hole Current
Electron remaining in the valence band are still attached to the atom – not free to move like free electron.
However, valence electron can move into nearby hole – leaving another hole it comes from.
Thus, hole has moved from one place to another.
The movement of electrons in a valence band is called hole current.
Figure 17: Hole current in intrinsic silicon.
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1.5 N-Type and P-Type Semiconductors
Doping
• Semiconductive materials do not conduct current well because of the limited
number of free electrons in the conduction band and holes in the valence band.
• Intrinsic semiconductive materials must be modified by increasing the free electrons
and holes to increase its conductivity and make it useful for electronic devices
– by adding impurities.
• Doping is the process of adding impurity atoms to intrinsic semiconductors improve its conductivity.
• Tw types of doping – N-Type and P-Type.
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1.5 N-Type and P-Type Semiconductors (cont.)
N-Type Semiconductor
• Is formed by adding pentavalent
(5 valence impurity atoms.
• To increase the number of free
electrons.
• 1 extra electrons becomes a
conduction electrons because it is
not attached to any atom.
• Pentavalent atom gives up
(donate) an electron –
call a donor atom. EKT 102: Basic Electronic Engineering
Figure 18: Pentavalent impurity atom in a silicon crystal structure. An antimony (Sb) impurity atom is shown in the center. The extra electron from the Sb atom becomes a free electron.
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1.5 N-Type and P-Type Semiconductors (cont.)
N-Type Semiconductor
• No. of conduction electrons can be controlled by the no. of impurity atoms.
• Since most of the current carriers are electrons, semiconductor doped with
pentavalent atoms is an n-type semiconductor.
• The electrons are called the majority carriers, while the holes is minority carriers.
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1.5 N-Type and P-Type Semiconductors (cont.)
P-Type Semiconductor
• Is formed by adding trivalent (3 valence impurity atoms.
• To increase the number of hole.
• A hole is created when each trivalent atom is added.
• Because the trivalent atom can take an electron, it is often
referred to as an acceptor atom.
• No. of holes can be controlled by the no. of trivalent impurity
atoms.
• Since most of the current carriers are holes, semiconductor
doped with trivalent atoms is an p-type semiconductor.
• The holes are called the majority carriers, while the
conduction electrons is minority carriers.
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Figure 19: Trivalent impurity atom in a siliconcrystal structure. A boron (B) impurity atom is shown in the center.